1. Field of the Invention:
This invention relates to a distance measuring device and an automatic focusing system using the distance measuring device. More particularly, the invention relates to a novel, infrared ray projecting type distance measuring device which is capable of reducing the need for an external circuit arrangement and achieving improvement in the tolerance of the system using the device. The invention also relates to an automatic focusing arrangement using this novel distance measuring device.
2. Description of the Prior Art:
Various kinds of distance measuring devices have been known and put to practical use. Of these conventional devices, active type infrared ray projecting distance measuring devices are more capable of measuring distances in dark place, because they are arranged to perform the trigonometric measurement by projecting infrared rays (or infrared light). The distance measuring device of this type, therefore, can be advantageously applied to an automatic focusing system of a camera or the like and is particularly advantageous for a camera of the type incorporating a flash device.
However, in order to detect the reflection of projected infrared light coming from an object whose distance is to be measured by isolating it from other disturbing light, it is necessary to carry out signal processing for synchronous detection, peak detection, etc. This results in a complex circuit arrangement which then makes it difficult to lessen external circuit arrangements even in cases where integrated circuits are employed.
In the accompanying drawings, FIG. 1 schematically shows the operating principle of conventional active, infrared ray projecting type distance measuring devices. The device shown comprises an infrared ray emitting diode 2 (hereinafter called IRED for short) which is arranged to project infrared light (or rays) to an object 6 through a light projecting optical system 4; and a light sensitive element 8 which receives reflected light coming from the object 6 through a light receiving optical system 10 and an infrared transmitting filter 12. With the device arranged in this manner, the IRED 2 is arranged to be moved by some suitable known means which is not shown, for example, in the direction of arrow A from a position a to another position b. The infrared light is projected along an optical path c when the IRED 2 is in the position a and along another optical path d when the IRED is in the other position b. Therefore, the intensity of the infrared rays to be sensed by the light sensitive element 8 becomes highest when the light is projected along an optical path e leading to the object 6 during the movement of the IRED 2. Assuming that the distance is to be measured on the basis of the principle of trigonometric distance measurement, the angle of projection, i.e. the moving position of IRED 2 at which the light sensitive element 8 most strongly senses the intensity of the infrared rays corresponds to a distance to the object. Since this operation is well known, further details of the operating principle of the conventional device are omitted.
The above description merely covers the principle of a conventional device. In actual applications of the device, however, the effect of external light cannot be ignored in spite of the use of the infrared transmitting filter 12. To solve this problem, in projecting the infrared light from the IRED 2, the infrared light is modulated while, at the light sensitive element 8, the reflected infrared light is detected through a synchronous detection process.
FIG. 2 shows, by way of example, a circuit arrangement to perform this synchronous detection. This circuit includes a light receiving amplifier 14 which is arranged to produce an output by converting a photo current flowing through the light sensitive element 8 into a voltage through a feedback resistor 16; a capacitor 18 which cuts the DC component of the output of the light receiving amplifier 14; a buffer amplifier which determines the level of the AC component after the DC component has been cut; a drive circuit 32 which modulates the IRED 2 by causing it to flicker at a frequency of about 10 KHz; a sample and hold circuit 22 which is arranged to sample and hold the output of the buffer amplifier 20 when the IRED 2 is lit; another sample and hold circuit 24 which is arranged to sample and hold the output of the buffer amplifier 20 when the IRED 2 is extinct and to produce an output by inverting it; an operational amplifier 26 which adds up the outputs of the sample and hold circuits 22 and 24; a capacitor 28 for a low-pass filter which cuts the high frequency component of the output of the operational amplifier 26; and a buffer amplifier 30 for producing an output. The sample and hold circuits 22 and 24 are controlled by the pulses produced by the drive circuit 32.
The operation of the circuit arrangement of the prior art device shown in FIG. 2 is as shown in the timing chart of FIG. 3. FIG. 3(1) shows the on and off timing of the IRED 2. FIG. 3(2) shows the output signal of the light receiving amplifier 14. FIG. 3(3) shows the output signal of the buffer amplifier 20. FIG. 3(4) shows the sampling pulses applied to the sample and hold circuit 22 by the drive circuit 32. FIG. 2(5) shows the sampling pulses applied to the sample hold and circuit 24 by the drive circuit 32. FIG. 3(6) shows the sample and hold output signal of the sample and hold circuit 22. FIG. 3(7) shows the sample and hold output signal of the sample and hold circuit 24. FIG. 3(8) shows the output signal of the operational amplifier 26. FIG. 3(9) shows the output signal of the buffer amplifier 30.
When reflected infrared rays strike the light sensitive element 8 according to the on-and-off operation of the IRED 2, a photo current in which an exterior light component and the reflected infrared light overlap flows through the light sensitive element 8. This photo current is voltage converted through the resistor 16 of the light receiving amplifier 14 into a voltage signal as shown in FIG. 3(2). The voltage signal then has the DC component thereof cut through the capacitor 18 and the buffer amplifier 20 and is taken out as an AC signal as shown in FIG. 3(3). Meanwhile, the sample and hold circuits 22 and 24 to which this AC signal is to be supplied respectively receive signal inputs as shown in FIG. 3(4) and (5). These signal inputs are supplied to the circuits 22 and 24 according to the lighting and extinction timing of the IRED 2 as sampling pulses respectively. Accordingly, as a result of their sample and hold actions, the circuits 22 and 24 respectively produce sample and hold signals as shown in FIGS. 3(6) and (7).
The outputs of the sample and hold circuits 22 and 24 are added at the operational amplifier 26. The output of the operational amplifier 26 is supplied to a lowpass filter with the waveform shown in FIG. 3(8). The lowpass filter which consists of the capacitor 28 and the buffer amplifier 30 removes a ripple component from the input to give a wave form as shown in FIG. 3(9).
Therefore, when the IRED is moved in the direction of the arrow A, the output of the buffer amplifier produces a signal which reaches a peak when light flux projected from the IRED 2 just impinge upon the distance measuring object 6. Then, it is possible either to measure a distance or to allow a camera or the like to perform an automatic focusing action by correlating the peak position thus obtained with the moving position of the IRED 2. However, in accordance with this arrangement, the DC component blocking capacitor 18, the capacitors in the circuits 22 and 24 used for sampling and holding and the capacitor 28 for the low-pass filter remain an external circuit even when the major part of the circuit arrangement is composed of integrated circuits. Besides, the arrangement to detect the peak of the output of the buffer amplifier 30 necessitates use of an additional capacitor for a peak holding action. In addition to that, the number of resistors required for adjustment must be also taken into account. Further, since signal processing is always accomplished by a real time operation, a data processing concept or an attempt to store the results of distance measurement cannot easily be carried out electrically but generally necessitates provision of either a mechanical converting arrangement or a memory arrangement.
Therefore, arrangement to connect a distance measuring device to the automatic focusing system of a camera or the like has unconditionally been limited to a connection arrangement in which an IRED and a lens are synchronously moved and the lens is brought to the stop at a peak position of infrared light received. This inevitably has limited the latitude allowable to the specification of a camera or the like.